Coolant circuit

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Chazbee
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Coolant circuit

Post by Chazbee »

Can someone tell me the pathway and direction of travel of the coolant through the engine,cylinder head and heater
in the following situations:-
a) When the thermostat is closed.
b) When the thermostat is open.
I haven’t got a problem - I’m just trying to straighten things out in my head! (I know - I should get out more).
Does the pump deliver into the block and suck from the bottom hose and bypass for example?
philthehill
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Re: Coolant circuit

Post by philthehill »

The coolant circulates thus:-
The coolant in the block/head heats up and when it gets to a specific temp set by the thermostat, the thermostat opens and the coolant flows/is pumped to the top of the radiator. As it cools the coolant sinks to the bottom of the radiator where it is picked up by the pump and pumped back to the block.
The pump and block design is such that coolant is preferentially pushed to the rear of the block. Coolant then passes to the head and then to the thermostat and the cycle starts again. There are transfer passages from block to head throughout the length of the block/head and water passes through them in varying volumes.
If the heater valve is open the pump pushes coolant from the rear of the head through the heater matrix and back to the bottom hose where it is drawn into the pump and re-circulated as above.
When the heater valve is shut coolant does not circulate via the heater matrix back to the bottom hose.

Chazbee
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Re: Coolant circuit

Post by Chazbee »

So before the thermostat opens,is the flow - through the block - to the cylinder head then down the bypass hose to the pump where it joins the heater return (assuming the heater valve is open) to be returned to the block?
philthehill
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Re: Coolant circuit

Post by philthehill »

A small amount of coolant passes down the bypass hose before the thermostat opens and is picked up by the pump and pushed back into circulation. The bypass hose allows a small amount of coolant to circulate so reducing the possibility of hot spots in the head before the thermostat opens.
The bypass hose has nothing to do with the heater coolant circuit.
Later 940 heads did not have the bypass facility but had a constant flow heater system which was regulated by a heater valve which allowed diverted coolant to reach the heater matrix if required.

Chazbee
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Re: Coolant circuit

Post by Chazbee »

I did’nt express myself very well regarding the heater return - I mean’t to say that I assumed that the returned heater water joins the bypass water in the pump inlet to be recirculated.
Your comments regarding the 940 head illustrates the necessity of keeping the heater valve open when there is no bypass for whatever reason - I would’nt think that a couple of small holes drilled in the thermostat would give enough
movement of coolant to avoid hotspots!
My last question (I promise) regarding the cooling system - what function does the sleeve sometimes fitted instead
of a thermostat have? Obviously running without a thermostat results in a very slow (not at all in Winter) warm up but presumably there is another important function of the sleeve.
Thanks for your patience Phil!!
philthehill
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Re: Coolant circuit

Post by philthehill »

The sleeve https://www.minispares.com/product/Clas ... o%20search when fitted instead of the thermostat is there to deflect the coolant to the back of the head where if any the hot spots occur.
I have gone back to fitting a 82 degree thermostat instead of the electronic thermostat and electric water pump because of the occurrence of hot spots in the head.
Drilling a few holes in the thermostat to allow a small amount of water to circulate before the thermostat opens does help but fitting a thermostat is the way to go plus the bypass hose fitment.
My own 940 head does not have the bypass facility but coolant exits the rear of the cylinder head via the heater outlet, goes to the oil/water intercooler and then joins the hose to the radiator (not pump otherwise I would be re-circulating hot coolant) so giving a continuous circulation of coolant.

Chazbee
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Re: Coolant circuit

Post by Chazbee »

Well I think I’m fully clued up on the cooling system now (thank goodness is the cry 😀😀). This forum is a fantastic
resource for us mere mortals especially when we’ve access to expertise from people such as Phil.
Thanks until the next question pops up in my head.
Sleeper
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Re: Coolant circuit

Post by Sleeper »

Ditto

Thanks
John ;-)
anthony22
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Re: Coolant circuit

Post by anthony22 »

Is it normal in this very cold weather for the heater to start warm and then go cold after about 3 miles and then warm up again.I have a 88c thermostat fitted.
No loss of water,I presume it is due to the thermostat opening and allowing the cold water to circulate.
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geoberni
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Re: Coolant circuit

Post by geoberni »

anthony22 wrote: Sat Jan 28, 2023 10:09 am Is it normal in this very cold weather for the heater to start warm and then go cold after about 3 miles and then warm up again.I have a 88c thermostat fitted.
No loss of water,I presume it is due to the thermostat opening and allowing the cold water to circulate.
Yes, the heater, when in circuit, bypasses the the Thermostat, as it comes from the top of the Block and returns to the bottom of the Radiator; so the Radiator coolant doesn't really change temp much at the start of the journey.
When the Thermostat opens, there's a rush of hot water to the top of the Radiator, which will undoubtedly lead to a few minutes of cooler water circulating...
Basil the 1955 series II

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Bill_qaz
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Re: Coolant circuit

Post by Bill_qaz »

As well as Phils detailed explanation you may find this interesting.
https://youtu.be/l1g2a8esM5E
Regards Bill
anthony22
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Re: Coolant circuit

Post by anthony22 »

Thank you for those replies,I only bought the car recently,the engine bay had evidence of rusty water that had sprayed everywhere.One of the core plugs was leaking,the one by the dipstick.

I will take it for a longer run now that it is working as it should.I have back flushed the old rusty water out ,new antifreeze,thermostat and new hoses.
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